1. Field of the Invention
This invention relates generally to bar graph display panels, and more particularly is directed to an improved control circuit therefor.
2. Description of the Prior Art
In order to display the level of an audio signal one well-known device now in use is referred to as a "bar graph". One such bar graph is manufactured by the Burroughs Corporation of Detroit, Mich. and is described in U.S. Pat. No. 3,973,166 issued Aug. 3, 1976 and entitled Display Panel for Displaying Bars of Light. As shown in the aforementioned patent, the bar graph display panel utilizes a gas-filled envelope containing a series of fine, closely-spaced cathode lines or elements connected in groups so that they can be energized sequentially and individually. The panel also includes an anode film aligned with the series of cathode lines and the panel contains ionizable gas. The display segments in such a panel are illuminated by a glow transfer principle. The glow is first established at a reset cathode and then transferred sequentially up to the desired height at a rate of approximately 60 cps, a rate at which a substantially flicker-free display is obtained. When the panel is energized, keep-alive electrodes connected to the power source provide excited particles for the reset cathode so that a glow discharge at the reset cathode is established to provide a visual display of analog signals. The height of the glow is determined in each scan by comparing a d.c. voltage representing the signal to be measured to an analog ramp voltage using a comparator the output of which is connected to the display panel anode such that when the d.c. voltage is the same or less than the ramp voltage, the anode voltage decreases below the ionization voltage and the loss of ionization voltage extinguishes the glow for the remainder of the scan.
Although the aforementioned bar graph and similar devices have been available for some time, various limitations have restricted their widespread use in the metering of signal levels. One basic limitation has been that the electronic circuits available for driving such a bar graph display panel have not been adequate. Such electronic circuits utilized a ramp generated by an R.C. network and such circuits did not permit intensified or brightened segments or what is referred to as "marker bars" to be obtained. Marker bars for such panels have been obtained but only by adding additional timing and clocking elements imposing further limitations in that such marker bars can be moved on the panel scale only by complex circuit changes.
It has long been sought to provide two level display utilizing such bar graphs if the bar graph display panel could be provided with several scales with different marker bar points and different available ranges. For example, if a display of the conventional European "Peak" scale per DIN is to be provided, a display from -40 db to +6 dB is desired with the dB steps in equally spaced increments and marker bars for each 10 dB is required. To display the American V.U. scale, the panel would have to be provided with a logarithmic scale from -20 dB to +3 dB with minus 10, -5, -3, 0, 1, 2 and 3 dB marker bars.
The following patents are representative of the prior art pertinent to the subject matter of this invention and all of which are clearly distinguishable from applicant's invention both structurally and functionally:
3,967,158 Saxon PA0 4,001,555 Levis et al. PA0 4,001,557 Stephenson
The Saxon patent utilizes a bar graph of the type described above in which an analog signal (to be displayed as a bar of light) and a ramp signal are coupled to a comparator the output of which energizes the anode electrode in the display panel. The cathode lines in the panel are energized or scanned and caused to glow sequentially so that when the two input signals, that is, the analog signal and the ramp signal, are equal, the output of the comparator is zero and anode is de-energized with the time in the scanning cyle when this occurs determining the number of cathodes which have been energized and the length of the bar of light displayed thereby. The basic concept in the Saxon patent is incorporated in bar graph display panels to which the applicant's invention is desirably applied but applicant's invention is directed specifically to a new and novel scan control circuit by means of which programmable memories (PROMs) are utilized for generating a reference ramp and for generating "marker bars" (intensified segments) at predefined positions on the bar graph display together with a circuit by means of which the ramp is generated in synchronism with the scanning of the cathodes in the display panel. The Stephenson patent relates to a digital process control function generator wherein a timing clock in combination with programmable read only memories controls variables in a process with respect to time. The Levis et al. patent relates to a signal processing apparatus for weighting an input signal in accordance with a mathematical transfer function to provide an output signal from the input signal as weighted by the function.